The stock prices of KeyCorp and Comerica had already lost more than 40 percent of their market value over the past year through the closing bell on Monday. KeyCorp was sporting a depressed share price of $10.89 at the close yesterday after trading in the single digits during the banking crisis in March. Then S&P Global delivered more bad news yesterday. It downgraded the credit rating on both KeyCorp and Comerica by one notch. Outlooks were indicated as “stable” for both banks by S&P…